xref: /dragonfly/lib/libc/db/man/btree.3 (revision c03f08f3) 
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28.\"	@(#)btree.3	8.4 (Berkeley) 8/18/94
29.\" $FreeBSD: src/lib/libc/db/man/btree.3,v 1.3.2.3 2003/03/15 15:11:05 trhodes Exp $
30.\" $DragonFly: src/lib/libc/db/man/btree.3,v 1.5 2007/08/18 20:48:47 swildner Exp $
31.\"
32.Dd August 18, 1994
33.Dt BTREE 3
34.Os
35.Sh NAME
36.Nm btree
37.Nd "btree database access method"
38.Sh LIBRARY
39.Lb libc
40.Sh SYNOPSIS
41.In sys/types.h
42.In db.h
43.Sh DESCRIPTION
44The routine
45.Fn dbopen
46is the library interface to database files.
47One of the supported file formats is
48.Nm
49files.
50The general description of the database access methods is in
51.Xr dbopen 3 ,
52this manual page describes only the
53.Nm
54specific information.
55.Pp
56The
57.Nm
58data structure is a sorted, balanced tree structure storing
59associated key/data pairs.
60.Pp
61The
62.Nm
63access method specific data structure provided to
64.Fn dbopen
65is defined in the
66.In db.h
67include file as follows:
68.Bd -literal
69typedef struct {
70	u_long flags;
71	u_int cachesize;
72	int maxkeypage;
73	int minkeypage;
74	u_int psize;
75	int (*compare)(const DBT *key1, const DBT *key2);
76	size_t (*prefix)(const DBT *key1, const DBT *key2);
77	int lorder;
78} BTREEINFO;
79.Ed
80.Pp
81The elements of this structure are as follows:
82.Bl -tag -width indent
83.It Va flags
84The flag value is specified by
85.Em or Ns 'ing
86any of the following values:
87.Bl -tag -width indent
88.It Dv R_DUP
89Permit duplicate keys in the tree, i.e. permit insertion if the key to be
90inserted already exists in the tree.
91The default behavior, as described in
92.Xr dbopen 3 ,
93is to overwrite a matching key when inserting a new key or to fail if
94the
95.Dv R_NOOVERWRITE
96flag is specified.
97The
98.Dv R_DUP
99flag is overridden by the
100.Dv R_NOOVERWRITE
101flag, and if the
102.Dv R_NOOVERWRITE
103flag is specified, attempts to insert duplicate keys into
104the tree will fail.
105.Pp
106If the database contains duplicate keys, the order of retrieval of
107key/data pairs is undefined if the
108.Va get
109routine is used, however,
110.Va seq
111routine calls with the
112.Dv R_CURSOR
113flag set will always return the logical
114.Dq first
115of any group of duplicate keys.
116.El
117.It Va cachesize
118A suggested maximum size (in bytes) of the memory cache.
119This value is
120.Em only
121advisory, and the access method will allocate more memory rather than fail.
122Since every search examines the root page of the tree, caching the most
123recently used pages substantially improves access time.
124In addition, physical writes are delayed as long as possible, so a moderate
125cache can reduce the number of I/O operations significantly.
126Obviously, using a cache increases (but only increases) the likelihood of
127corruption or lost data if the system crashes while a tree is being modified.
128If
129.Va cachesize
130is 0 (no size is specified) a default cache is used.
131.It Va maxkeypage
132The maximum number of keys which will be stored on any single page.
133Not currently implemented.
134.\" The maximum number of keys which will be stored on any single page.
135.\" Because of the way the
136.\" .Nm
137.\" data structure works,
138.\" .Va maxkeypage
139.\" must always be greater than or equal to 2.
140.\" If
141.\" .Va maxkeypage
142.\" is 0 (no maximum number of keys is specified) the page fill factor is
143.\" made as large as possible (which is almost invariably what is wanted).
144.It Va minkeypage
145The minimum number of keys which will be stored on any single page.
146This value is used to determine which keys will be stored on overflow
147pages, i.e. if a key or data item is longer than the pagesize divided
148by the minkeypage value, it will be stored on overflow pages instead
149of in the page itself.
150If
151.Va minkeypage
152is 0 (no minimum number of keys is specified) a value of 2 is used.
153.It Va psize
154Page size is the size (in bytes) of the pages used for nodes in the tree.
155The minimum page size is 512 bytes and the maximum page size is 64K.
156If
157.Va psize
158is 0 (no page size is specified) a page size is chosen based on the
159underlying file system I/O block size.
160.It Va compare
161Compare is the key comparison function.
162It must return an integer less than, equal to, or greater than zero if the
163first key argument is considered to be respectively less than, equal to,
164or greater than the second key argument.
165The same comparison function must be used on a given tree every time it
166is opened.
167If
168.Va compare
169is
170.Dv NULL
171(no comparison function is specified), the keys are compared
172lexically, with shorter keys considered less than longer keys.
173.It Va prefix
174The
175.Va prefix
176element
177is the prefix comparison function.
178If specified, this routine must return the number of bytes of the second key
179argument which are necessary to determine that it is greater than the first
180key argument.
181If the keys are equal, the key length should be returned.
182Note, the usefulness of this routine is very data dependent, but, in some
183data sets can produce significantly reduced tree sizes and search times.
184If
185.Va prefix
186is
187.Dv NULL
188(no prefix function is specified),
189.Em and
190no comparison function is specified, a default lexical comparison routine
191is used.
192If
193.Va prefix
194is
195.Dv NULL
196and a comparison routine is specified, no prefix comparison is
197done.
198.It Va lorder
199The byte order for integers in the stored database metadata.
200The number should represent the order as an integer; for example,
201big endian order would be the number 4,321.
202If
203.Va lorder
204is 0 (no order is specified) the current host order is used.
205.El
206.Pp
207If the file already exists (and the
208.Dv O_TRUNC
209flag is not specified), the
210values specified for the
211.Va flags , lorder
212and
213.Va psize
214arguments
215are ignored
216in favor of the values used when the tree was created.
217.Pp
218Forward sequential scans of a tree are from the least key to the greatest.
219.Pp
220Space freed up by deleting key/data pairs from the tree is never reclaimed,
221although it is normally made available for reuse.
222This means that the
223.Nm
224storage structure is grow-only.
225The only solutions are to avoid excessive deletions, or to create a fresh
226tree periodically from a scan of an existing one.
227.Pp
228Searches, insertions, and deletions in a
229.Nm
230will all complete in
231O lg base N where base is the average fill factor.
232Often, inserting ordered data into
233.Nm Ns s
234results in a low fill factor.
235This implementation has been modified to make ordered insertion the best
236case, resulting in a much better than normal page fill factor.
237.Sh ERRORS
238The
239.Nm
240access method routines may fail and set
241.Va errno
242for any of the errors specified for the library routine
243.Xr dbopen 3 .
244.Sh SEE ALSO
245.Xr dbopen 3 ,
246.Xr hash 3 ,
247.Xr mpool 3 ,
248.Xr recno 3
249.Rs
250.%T "The Ubiquitous B-tree"
251.%A Douglas Comer
252.%J "ACM Comput. Surv. 11"
253.%N 2
254.%D June 1979
255.%P 121-138
256.Re
257.Rs
258.%A Bayer
259.%A Unterauer
260.%T "Prefix B-trees"
261.%J "ACM Transactions on Database Systems"
262.%N 1
263.%V Vol. 2
264.%D March 1977
265.%P 11-26
266.Re
267.Rs
268.%B "The Art of Computer Programming Vol. 3: Sorting and Searching"
269.%A D. E. Knuth
270.%D 1968
271.%P 471-480
272.Re
273.Sh BUGS
274Only big and little endian byte order is supported.
275